Unless I planned to call gobblers up very close I would not use lead 7 1/2 shot. It will give you a very dense pattern because of the large number of pellets in the shell but it runs out of penetrating power too quickly.

In lead shot I would use either 5's or 6's. A good rule of thumb for determining your maximum range is the longest distance that your gun will put 80 to 100 pellets inside a 10 inch circle. Even though the individual pellets of number 4 shot have the power to kill way out there it's a rare gun that will give you those numbers at 40 yards with lead 4's. Lethal killing energy is meaningless without a pattern dense enough to insure consistent hits to the head or spine. If number 5's would give me those numbers at 40 yards then that is what I would shoot. If not then I'd use 6's to get a denser pattern. But be aware that despite what you may hear on the internet 35 to 40 yards is about the max lethal power range for lead 6's no matter how far out they yield the pattern numbers I mentioned. Some would firmly say 35 yards is the max for lead 6's.

This is where the beauty of denser than lead shot comes in. First it just patterns better than lead period. Second you can shoot denser than lead 6's or 7's which have a lot of pellets to boost pattern density without sacrificing lethal range like you do with lead 6's or 7's. Why? Because the denser than lead stuff also out penetrates lead by a wide margin.

I shot a gobbler a while back with Winchester Extended Range 6's. Their version of a denser than lead shell. He was standing at exactly 40 yards with his left side to-wards me. I shot and dropped him. When I got him home and cleaned him I noticed that some of the pellets in the lower part of the pattern had passed through his left wing, through his left breast feathers, through the meat of his left breast, through the breast plate bone dividing the two breasts and were lodged halfway through the meat of his right breast. And that was fired from a shotgun with a 20 inch barrel. That is not just a little better penetration than lead 6's, that is in a whole other world of penetration. It's also why I will never go back to lead.